Re: [PATCH] libata: rewrite SCSI host scheme to be one per ATA host

Jeff Garzik
2009-04-22T16:48:11Z

Boaz Harrosh wrote:
> On 04/22/2009 12:23 PM, Jeff Garzik wrote:
>> Jeff Garzik wrote:
>>> Currently, libata creates a Scsi_Host per port. This was originally
>>> done to leverage SCSI's infrastructure to arbitrate among master/slave
>>> devices, but is not needed for most modern SATA controllers. And I
>>> _think_ it is not needed for master/slave if done properly, either.
>> BTW note the above, with regards to the libata SCSI->block conversion.
>> libata currently relies on SCSI for some amount of generic device
>> arbitration, in several situations (see ->qc_defer,
>> SCSI_MLQUEUE_.*_BUSY). libata expects SCSI to be intelligent and not
>> starve devices, etc.
>>
>>
>>> I was able to successfully boot the following patch on
>>> AHCI/x86-64/Fedora.
>>>
>>> It may work with other controllers -- TRY AT YOUR OWN RISK. It will
>>> probably fail for master/slave configurations, and SAS & PMP also
>>> need looking at. It yielded this lsscsi output on my AHCI box:
>>>
>>> [0:0:0:0] disk ATA ST3500320AS SD15 /dev/sda
>>> [0:2:0:0] disk ATA G.SKILL 128GB SS 02.1 /dev/sdb
>>> [0:5:0:0] cd/dvd PIONEER BD-ROM BDC-202 1.04 /dev/sr0
>> For comparison, here is unmodified 2.6.30-rc3:
>>
>> [jgarzik@bd ~]$ lsscsi
>> [0:0:0:0] disk ATA ST3500320AS SD15 /dev/sda
>> [2:0:0:0] disk ATA G.SKILL 128GB SS 02.1 /dev/sdb
>> [5:0:0:0] cd/dvd PIONEER BD-ROM BDC-202 1.04 /dev/sr0
>>
>
> Could the master/slave be simply solved by emulating a SCSI LUN
> for example below is my machine today:
> []$ lsscsi
> [0:0:0:0] disk ATA ST3160023A 3.01 /dev/sda
> [1:0:0:0] cd/dvd _NEC DVD_RW ND-3550A 1.05 /dev/sr0
> [4:0:0:0] disk ATA WDC WD1600JS-60M 10.0 /dev/sdb
>
> the /dev/sda and /dev/sr0 share a master/slave wide cable (sdb is sata)
>
> it could be made to scan as:
> []$ lsscsi
> [0:0:0:0] disk ATA ST3160023A 3.01 /dev/sda
> [0:0:0:1] cd/dvd _NEC DVD_RW ND-3550A 1.05 /dev/sr0
> [1:0:0:0] disk ATA WDC WD1600JS-60M 10.0 /dev/sdb
>
> So we need to emulate the REPORT_LUN (or what ever else) to return
> two LUNs. Or do you want to report a separate target for the master/slave?

Mapping master/slave is not difficult -- each should be a separate
target, just like with parallel SCSI.

The issue with master/slave and simplex is guaranteeing that only _one_
command may be executing at a time, for a given set of targets, i.e.
only one command per master/slave pair, only one command per pair of
simplex ports (== 4 ATA devices max).

Originally this was done by setting can_queue==1, cmd_per_lun==1, and
assigning each master/slave pair to a different Scsi_Host.

Jeff